The present invention relates to a method of and an apparatus for detecting a reduction in the air pressure of each of the tires of a four-wheeled vehicle, and more particularly to a tire air-pressure reduction detecting method capable of securely detecting a reduction in air pressure even though the vehicle is under travelling, and also to a tire air-pressure reduction detecting apparatus using this method.
As one of safety devices for a four-wheeled vehicle such as a passenger car, a truck or the like, an apparatus for detecting a reduction in the air pressure of a tire has recently been invented and partially put in practical use.
The tire air-pressure reduction detecting apparatus has been developed with its importance recognized mainly for the following reasons. If a tire is low in air pressure, the tire is increased in flexure which can raise the tire temperature. When the tire temperature is raised, a polymeric material used for the tire is lowered in strength. This may provoke tire bursting. However, even though a tire is reduced in air pressure, the driver is not aware of such a reduction.
In such a detecting apparatus, there may be used, for example, a method of detecting a reduction in air pressure based on differences among the rotational angular speeds F.sub.1, F.sub.2, F.sub.3, F.sub.4 (hereinafter collectively referred to as "rotational angular speeds F.sub.i ") of the four tires W.sub.1, W.sub.2, W.sub.3, W.sub.4 (in which the tires W.sub.1, W.sub.2 respectively correspond to the front left and right wheels, while the tires W.sub.3, W.sub.4 respectively correspond to the rear left and right wheels, and which are hereinafter collectively referred to as "tires W.sub.i ").
According to this method, the rotational angular speeds F.sub.i of the tires W.sub.i are detected per predetermined sampling period based on signals supplied from wheel speed sensors attached to the tires W.sub.i for example. When the dynamic load radii of the tires W.sub.i (the apparent rolling radii of the tires as calculated by dividing, by 2.pi., the distances that the vehicle advances during one rotation of the tires when the vehicle travels) are the same as one another, the rotational angular speeds F.sub.i are the same as one another as far as the vehicle linearly travels.
On the other hand, the dynamic load radius of a tire W.sub.i varies with, for example, a change in the air pressure of the tire W.sub.i. The dynamic load radius of a tire W.sub.i reduced in air pressure becomes smaller than that of a tire normal in inner pressure. Accordingly, the rotational angular speed F.sub.i of this tire W.sub.i is faster than that of a tire normal in inner pressure. It is therefore possible to detect a reduction in the air pressure of any of the tires W.sub.i based on differences in rotational angular speed F.sub.i. The following equation (1) shows a judging formula for detecting a reduction in the air pressure of any of the tires W.sub.i (See British Patent Publication No. GB-8711310 (A) and British Patent Publication No. GB-902925 (A)): ##EQU1##
For example, when the dynamic load radii of the tires W.sub.i are supposed to be the same as one another, the rotational angular speeds F.sub.i are the same as one another (F.sub.1 =F.sub.2 =F.sub.3 =F.sub.4) and the judgement value D becomes 0. Here, there are determined threshold values D.sub.TH1, D.sub.TH2 (each of D.sub.TH1, D.sub.TH2 is greater than 0). When D satisfies the following judging formula: EQU D&lt;-D.sub.TH1 or D&gt;D.sub.TH2 (2)
it is judged that there is a tire W.sub.i reduced in air pressure. When D does not satisfy the judging formula, it is judged that there is no tire W.sub.i reduced in air pressure.
On the other hand, even though the tires W.sub.1 to W.sub.4 are normal in inner pressure, the dynamic load radii are not always the same as one another. That is, the tires W.sub.i are produced as always containing variations (hereinafter referred to as "initial differences") within the standards. It is known that the degree of such variations is about 0.1% in terms of standard deviation. For example, when a tire W.sub.i is reduced in air pressure by 0.6 kg/cm.sup.2 (reduction by 30% when the normal inner pressure is 2.0 kg/cm.sup.2), the variation of the dynamic load radius is about 0.2% of the normal inner pressure. The variation of a dynamic load radius due to initial difference is substantially the same as the variation of the dynamic load radius due to a reduction in air pressure. Accordingly, a difference in rotational angular speed F.sub.i due to initial difference is substantially the same as a difference in rotational angular speed F.sub.i due to reduction in air pressure. Accordingly, there are instances where the inner pressures are normal even though the judgement value D is not 0. It is therefore not possible to securely detect a reduction in air pressure only by a method of using the judgement value D on the basis of 0.
Further, when a tire is replaced or air is replenished thereinto, there is generated a change which can be regarded as an error corresponding to the initial difference. In such a case, too, a reduction in air pressure cannot be detected accurately.
To solve the problem above-mentioned, it is required, before detecting a reduction in air pressure, to previously execute a processing of obtaining coefficients for correcting the initial differences (hereinafter referred to as "the initial correction processing"). The initial correction processing is for example discussed in Japanese Patent Application No. 4-246848 (Japanese Laid-Open Patent Application No. 6-92114/1994; laid open to the public on Apr. 5, 1994) which the applicant of the present application has previously filed. According to this processing, the vehicle is linearly travelled at a predetermined speed when it is known that all the tires W.sub.1 to W.sub.4 are normal in inner pressure, the rotational angular speeds F.sub.i are calculated during this travelling, and there are obtained correction coefficients C.sub.ni based on a certain tire W.sub.i. For example, when the tire W.sub.1 is used as a basis, the correction coefficients C.sub.ni are obtained according to the following equations: EQU C.sub.n1 =F.sub.1 /F.sub.1 ( 3) EQU C.sub.n2 =F.sub.1 /F.sub.2 ( 4) EQU C.sub.n3 =F.sub.1 /F.sub.3 ( 5) EQU C.sub.n4 =F.sub.1 /F.sub.4 ( 6)
These correction coefficients C.sub.ni are stored in a nonvolatile memory. When the correction coefficients C.sub.ni are respectively multiplied by rotational angular speeds F.sub.i calculated at the time the vehicle actually travels later, the initial differences of the tires W.sub.i can be corrected.
However, the tire air-pressure reduction detecting method above-mentioned presents the following problems (A) to (C).
(A) As shown by the equations (5), (6), each of the correction coefficients C.sub.n3, C.sub.n4 for the tires W.sub.3, W.sub.4 is obtained from the ratio of the rotational angular speed F.sub.i of the front tire to the rotational angular speed F.sub.i of the rear tire. For a two wheel drive vehicle (2WD) for example, such a correction coefficient C.sub.ni is obtained from the ratio between the rotational angular speed of a non-driving tire and the rotational angular speed of a driving tire (hereinafter referred to as "the front/rear wheel ratio"). When the equations (5), (6) are taken as examples, the front/rear wheel ratio is equivalent to the ratio of the rotational angular speed of a driving tire to the rotational angular speed of a non-driving tire when the vehicle is of the front-wheel drive type (FWD), and the front/rear wheel ratio is equivalent to the ratio of the rotational angular speed of a non-driving tire to the rotational angular speed of a driving tire when the vehicle is of the rear-wheel drive type (RWD).
On the other hand, the calculated rotational angular speeds F.sub.i naturally vary with the vehicle speed. However, such variations are different between the driving tires to which torque is applied, and the non-driving tires to which no torque is applied. That is, torque is applied to the driving tires and the driving tires are accordingly liable to slip with an increase in vehicle speed or forward/backward acceleration. When the driving tires slip, the driving tires are increased in rotational angular speed, as compared with the non-driving tires. Thus, the front/rear wheel ratio varies with the speed and forward/backward acceleration.
When the vehicle is of front-wheel drive type, variations of the front/rear wheel ratio with respect to the speed are shown in FIG. 10, while variations of the front/rear wheel ratio with respect to the forward/backward acceleration are shown in FIG. 11.
In the initial correction processing, the correction coefficients C.sub.ni are obtained based on the rotational angular speeds F.sub.i calculated at a predetermined speed, and then stored in a nonvolatile memory. Accordingly, the correction coefficients C.sub.ni are always used when the vehicle actually travels. However, the correction coefficients C.sub.n3, C.sub.n4 correspond to the front/rear wheel ratio. Accordingly, the correction coefficients C.sub.n3, C.sub.n4 actually vary with the speed. Thus, when the fixed correction coefficients C.sub.n3, C.sub.n4 are used while the vehicle actually travels, it is not possible to execute accurate initial correction, resulting in a failure to detect a reduction in air pressure with high precision.
(B) It is known that the dynamic load radius of a tire W.sub.i is increased with an increase in vehicle speed (this is called tread lifting)(See FIG. 12). That is, a centrifugal force to be applied to a tire W.sub.i is increased with an increase in vehicle speed. However, this tread lifting presents the problem that a reduction in air pressure is readily detected when the vehicle travels at a low speed, but is detected with much difficulty when the vehicle travels at a high speed.
More specifically, a centrifugal force applied to a tire W.sub.i reduced in air pressure shown in FIG. 13(a) is small while the vehicle travels at a low speed. Accordingly, the dynamic load radius R1 is smaller than the dynamic load radius R2 of a tire W.sub.i normal in air pressure shown in FIG. 13(b). Thus, while the vehicle travels at a low speed, there is a remarkable difference in rotational angular speed F.sub.i between the tire W.sub.i reduced in air pressure and the tire W.sub.i normal in inner pressure. Accordingly, the judgement value D becomes relatively great, enabling a reduction in air pressure to be readily detected.
On the other hand, while the vehicle travels at a high speed, a great centrifugal force is applied to the tires. Accordingly, as shown in FIG. 14 (a), the dynamic load radius R1 of the tire W.sub.i reduced in air pressure shown in FIG. 13(a) becomes great substantially as much as the dynamic load radius R2 of a tire W.sub.i normal in inner pressure shown in FIG. 14(b). On the contrary, tension exerted to the tire W.sub.i normal in inner pressure shown in FIG. 14(b) is greater than that of a tire reduced in air pressure. Therefore, the tire Wi normal in inner pressure is unaffected so much by a centrifugal force, even if the vehicle travels at a high speed. As a result, the dynamic load radius R2 of the tire W.sub.i normal in inner pressure undergoes no substantial change. Accordingly, while the vehicle travels at a high speed, there is substantially no difference in rotational angular speed F.sub.i between the tire W.sub.i reduced in air pressure and the tire W.sub.i normal in inner pressure. Accordingly, the judgement value D approaches zero very much. Thus, the judgement value D does not satisfy the judging formula (2). This involves the likelihood that a reduction in air pressure cannot securely be detected. That is, while the vehicle travels at a high speed, a reduction in air pressure cannot securely be detected even though a tire is actually reduced in air pressure as done while the vehicle travels at a low speed.
(C) It is known that, due to the vehicle suspension having the tires W.sub.1 to W.sub.4, the characteristics of a fluid or a spring used therein, the amounts of changes in the dynamic load radii of the tires W.sub.1 to W.sub.4 resulting from reductions in air pressure, are different from one another dependent on the attachment positions of the tires W.sub.i, even though the reduction rates are the same as one another. That is, the front-left tire and the rear-right tire are different from each other in the change amount of dynamic load radius. As a result, even though the tires W.sub.1 to W.sub.4 are reduced in air pressure at the same rate, there are generated differences in rotational angular speed F.sub.i among the tires W.sub.1 to W.sub.4. Thus, the judgement values D obtained according to the equation (1) are different from one another dependent on which tires are reduced in air pressure.
FIG. 15 shows the absolute value of judgement values D obtained at the time when the tires W.sub.1 to W.sub.4 are individually depressurized. In FIG. 15, when the threshold value D.sub.TH2 used in the judging formula (2) is set to .alpha. (the threshold value cannot be set too small because there are instances where the judgement value D becomes a value other than 0 due to reasons other than a reduction in air pressure), a reduction in air pressure of each of the tires W.sub.3, W.sub.4 can be detected, but a reduction in air pressure of each of the tires W.sub.1, W.sub.2 cannot be detected. This presents the problem that, even though a tire W.sub.i reduced in air pressure actually exists, such a reduction may not be detected dependent on the position of the tire W.sub.i. This is not preferable in view of traffic safety.